Method of using alkali metal compounds and organic solvents to activate red phosphorous for synthesizing soluble polyphosphides

ABSTRACT

A method of using alkali metal compounds and organic solvents to activate red phosphorous for synthesizing soluble polyphosphides is presented. A reaction between an alkali metal alkoxy compound or alkali metal alkyl thiolate compound, such as potassium ethoxide, and shelf stable red phosphorus generated soluble polyphosphides in a variety of organic solvents with the  31 P NMR spectrum being used to detect the species of polyphosphides produced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to currentlypending U.S. Nonprovisional Application Ser. No. 15/428,314, entitled“Method of Conversion of Red Phosphorous to Soluble Polyphosphides”,filed Feb. 9, 2017 which claims priority to U.S. Provisional PatentApplication No. 62/292,985, entitled “Solubilization of Red Phosphorusby Reaction with Potassium Ethoxide”, filed Feb. 9, 2016, the entiretyof which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to chemical reactions. More specifically, theinvention relates to chemical methodology for conversion of redphosphorus to polyphosphides via solution-chemistry routes.

BACKGROUND OF THE INVENTION

Polyphosphides are negatively charged clusters of phosphorus atoms thatexhibit multifarious structural motifs. The majority of polyphosphideshad been obtained by direct solid-state reactions between metals and redphosphorus red, (P_(red)) or by iodine-assisted chemical vapor transport(von Schnering & Hönle, Chemistry and structural chemistry of phosphidesand polyphosphides. 48. Bridging chasms with polyphosphides. Chem. Rev.,1988 January; 88 (1): 243-273; Pöttgen, W. Hönle, H. G. von Schnering inEncyclopedia of Inorganic Chemistry, Vol. 8, 2^(nd) ed. (Ed.: R. B.King), Wiley, Chichester, 2005, p. 4268). Only a handful ofuncoordinated polyphosphide anions have been obtained by solution-basedmethods (Baudler, Polyphosphorus Compounds—New Results and Perspectives.Angew. Chem. Int. Ed. Engl. 1987 May; 26(5), 419-441; Baudler, et al.,Trilithium Heptaphosphide, Dilithium Hexadecaphosphide, and TrisodiumHenicosaphosphide. Inorg. Synth. 1990, 27, 227; von Schnering, et al.,Chemistry and structural chemistry of phosphides and polyphosphides. 28.Bis(tetraphenylphosphonium) hexadecaphosphide, a salt with the newpolycyclic anion P162. Angew. Chem. Int. Ed. Engl. 1981, 20: 594;Miluykov, et al., Facile routes to sodium tetradecaphosphide Na₄P₁₄ andmolecular structure of Na₄(DME)_(7.5)P₁₄ and Na₄(en)₆P₁₄(DME=1,2-dimethoxyethane; en=ethylenediamine). Z. Anorg. Allg. Chem.2006, 632(10-11): 1728-32), which can be explained by the difficulty inisolating these species. In general, the highly reactive polyphosphidefragments need to be captured with organic or organometallic reagents.

The long history of polyphosphides notwithstanding, there is currently agrowing interest in the study of these species. Research efforts inpolyphosphide chemistry have been fueled by aspirations to control theactivation of the P₄ molecule (Cossairt, et al.,Early-transition-metal-mediated activation and transformation of whitephosphorus. Chem. Rev. 2010 Jul. 14, 110(7), 4164-77; Cummins, Terminal,anionic carbide, nitride, and phosphide transition-metal complexes assynthetic entries to low-coordinate phosphorus derivatives. Angew. Chem.Int. Ed. Eng. 2006 Jan. 30, 45(6), 862-70), known as the white allotropeof the element (P_(white)), and by the recent discovery of phosphoreneas a promising graphene analogue with a finite direct band gap (Reich,Phosphorene excites materials scientists. Nature. 2014 Feb. 6,506(7486), 19; Xia, et al., Rediscovering black phosphorus as ananisotropic layered material for optoelectronics and electronics. Nat.Commun. 2014 Jul. 21; 5: 4458; Liu, et al., Phosphorene: an unexplored2D semiconductor with a high hole mobility. ACS Nano. 2014 Apr. 22,8(4), 4033-4041; Li, et al., Black phosphorus field-effect transistors.Nat. Nanotechnol. 2014 May, 9(5), 372-377; Liu, et al., The effect ofdielectric capping on few-layer phosphorene transistors: tuning theSchottky barrier heights IEEE Electron. Device Lett. 2014 May, 35(7),795-797).

Most polyphosphides prepared by solid-state methods are insoluble incommon organic solvents and exhibit very high chemical stability (vonSchnering & Hönle, Chemistry and structural chemistry of phosphides andpolyphosphides. 48. Bridging chasms with polyphosphides. Chem. Rev.,1988 January; 88 (1): 243-273; Bawoh & Nilges, Phosphorus Rich d¹⁰ IonPolyphosphides and Selected Materials. Z. Anorg. Allg. Chem. 2015,641(2), 304-310) In contrast, the solution methods furnish soluble andreactive polyphosphide fragments, many of which were not detected in thesolid-state reactions. Therefore, the need for a broader exploration ofsolution-based routes cannot be overstated, as these synthetic methodsprovide access to different polyphosphide species as a result ofkinetically controlled, rather than thermodynamically controlled,reaction pathways. Not only do these species exhibit fascinatingreactivity (Turbervill & Goicoechea, From clusters to unorthodoxpnictogen sources: solution-phase reactivity of [E₇]³⁻ (E=P—Sb) anions.Chem. Rev. 2014 Nov. 12, 114(21), 10807-10828) but they also might serveas precursors for high-performance materials, including 2Dsemiconductors (Reich, Phosphorene excites materials scientists. Nature.2014 Feb. 6, 506(7486), 19; Xia, et al., Rediscovering black phosphorusas an anisotropic layered material for optoelectronics and electronics.Nat. Commun. 2014 Jul. 21; 5: 4458; Liu, et al., Phosphorene: anunexplored 2D semiconductor with a high hole mobility. ACS Nano. 2014Apr. 22, 8(4), 4033-4041; Li, et al., Black phosphorus field-effecttransistors. Nat. Nanotechnol. 2014 May, 9(5), 372-377; Liu, et al., Theeffect of dielectric capping on few-layer phosphorene transistors:tuning the Schottky barrier heights IEEE Electron. Device Lett. 2014May, 35(7), 795-797) and lithium-ion battery anodes (Wang, et al.,Nano-Structured Phosphorus Composite as High-Capacity Anode Materialsfor Lithium Batteries. Angew. Chem. Int. Ed. 2012 September;51(36):9034-9037).

The majority of solution-based routes for producing polyphosphidesemploy the toxic and flammable P_(white) allotrope, strongly reducingconditions, and/or cryogenic solvents (Baudler, PolyphosphorusCompounds—New Results and Perspectives. Angew. Chem. Int. Ed. Engl. 1987May; 26(5), 419-441; Baudler, et al., Trilithium Heptaphosphide,Dilithium Hexadecaphosphide, and Trisodium Henicosaphosphide. Inorg.Synth. 1990, 27, 227; von Schnering, et al., Chemistry and structuralchemistry of phosphides and polyphosphides. 28.Bis(tetraphenylphosphonium) hexadecaphosphide, a salt with the newpolycyclic anion P162. Angew. Chem. Int. Ed. Engl. 1981, 20: 594;Miluykov, et al., Facile routes to sodium tetradecaphosphide Na₄P₁₄ andmolecular structure of Na₄(DME)_(7.5)P₁₄ and Na₄(en)₆P₁₄(DME=1,2-dimethoxyethane; en=ethylenediamine). Z. Anorg. Allg. Chem.2006, 632(10-11): 1728-32). Such methods, therefore, are difficult toscale up, which limits the potential uses of polyphosphides and hindersmore extensive studies of their reactivity. There have been a fewreports whereby P_(red) was used to prepare species such as K₃P₇, butthe solvents were limited to liquid ammonia or ethylenediamine incombination with strongly reducing agents (Na or Na—K alloy) (Miluykov,et al., Facile routes to sodium tetradecaphosphide Na₄P₁₄ and molecularstructure of Na₄(DME)_(7.5)P₁₄ and Na₄(en)₆P₁₄ (DME=1,2-dimethoxyethane;en=ethylenediamine). Z. Anorg. Allg. Chem. 2006, 632(10-11): 1728-32;Schmidbaur & Bauer, An improved preparation oftris(trimethylsilyl)heptaphosphine. Phosphorus Sulfur Silicon Relat.Elem. 1995, 102(1-4), 217-219).

As such, there is a deficiency in the art to produce solublepolyphosphides from red phosphorous. As noted above, the difficulties inthe art with respect to solubilizing P_(red), along with the highlytoxic materials used to solubilize P_(red), evidence an unmet need inthe art for methods of dissolving or solubilizing P_(red) orpolyphosphides thereof in a liquid medium.

BRIEF SUMMARY OF THE INVENTION

Red phosphorus is solubilized by subjecting red phosphorus or acomposition of black phosphorus and red phosphorus to an alkali metalalkoxy compound or alkali metal alkyl thiolate compound suspended in anorganic solvent, to produce polyphosphide anions soluble in organicsolvents. The alkali metal is optionally sodium or potassium.Nonlimiting examples of alkali metal alkoxy compound or alkali metalalkyl thiolate compounds include potassium ethoxide, potassiummethoxide, sodium ethoxide, sodium methoxide, and sodium methylthiolate. Optional, nonlimiting examples of organic solvents includemethyl cyanide, dimethyl sulfoxide, dimethylformamide, or a combinationof tetrahydrofuran and dimethoxyethane. In specific variations, theorganic solvent is a combination of tetrahydrofuran and dimethoxyethaneat a volume to volume ratio of 1:1.

The red phosphorus and alkali metal or alkali metal compound are reactedvia reflux or an in-line packed column method. Where the reaction occursvia reflux, the reflux is performed for about 15 minutes to about 2hours. Nonlimiting examples include 15 minutes, 20 minutes, 25 minutes,30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes,1 hour, 1.1 hours, 1.2 hours, 1.25 hours, 1.3 hours, 1.4 hours, 1.5hours, 1.6 hours, 1.7 hours, 1.75 hours, 1.8 hours, 1.9 hours, 2 hours,2.1 hours, and 2.2 hours.

In variations using an in-line packed method, the red phosphorus orcomposition of black phosphorus and red phosphorus is loaded into apacking column either in an inert environment or in ambient environmentand flushed with an inert gas. Optionally, the red phosphorus orcomposition of black phosphorus and red phosphorus has a grain size ofabout 1.4 mm to about 0.7 mm. Nonlimiting examples of grain sizesinclude 0.6 mm, 0.65 mm, 0.7 mm, 0.75 mm, 0.8 mm, 0.85 mm, 0.9 mm, 0.95mm, 1.0 mm, 1.05 mm, 1.1 mm, 1.15 mm, 1.2 mm, 1.25 mm, 1.3 mm, 1.35 mm,1.4 mm, 1.45 mm, 1.5 mm, 1.55 mm, and 1.6 mm. The pressure on thepacking column is set to at least 1 bar. Nonlimiting examples ofpressures include 1 bar, 1.5 bar, 2 bar, 2.5 bar, 3 bar, 3.5 bar, 4 bar,4.5 bar, 5 bar, 5.5 bar, 6 bar, 6.5 bar, 7 bar, 7.5 bar, 8 bar. Thepacking column is heated to at least the boiling point of the alkalimetal alkoxy compound or alkali metal alkyl thiolate. Nonlimitingexamples of temperatures include about 80° C. to about 400° C. Specific,nonlimiting examples include 80° C., 85° C., 90° C., 95° C., 100° C.,110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C.,190° C., 200° C., 210° C., 220° C., 230° C., 240° C., 250° C., 260° C.,270° C., 280° C., 290° C., 300° C., 310° C., 320° C., 330° C., 340° C.,350° C., 360° C., 370° C., 380° C., 390° C., and 400° C. In specificvariations, the temperature of the packing column or a reaction solutiondoes not exceed the critical temperature of the organic solvent. Thealkali metal alkoxy compound or alkali metal alkyl thiolate compound issuspended in the organic solvent and then flowed through the packingcolumn to form a compound containing the alkali metal and polyphosphideanions. The compound containing the alkali metal and polyphosphideanions is optionally collected and optionally further processed.Alternatively, the black phosphorus is separated from the polyphosphideanions.

In certain variations, the packing column is stainless steel, castaluminum, A10 aluminum, A12 aluminum, stainless steel, aluminum-nickelalloy (50-50), polyimide, polyethylene terephthalate, polyamide-imide,nylon, polyvinyl chloride, polycarbonate, polyphthalamide, polysulfone,maleimide/bismaleimide, polyetheretherketone, polyetherimide, polyimide,polyester, acrylonitrile butadiene styrene, carbon reinforced-polyimide,carbon reinforced-polyethylene terephthalate, carbonreinforced-polyamide-imide, carbon reinforced-nylon, carbonreinforced-polyvinyl chloride, carbon reinforced-polycarbonate, carbonreinforced-polyphthalamide, carbon reinforced-polysulfone, carbonreinforced-maleimide/bismaleimide, carbonreinforced-polyetheretherketone, carbon reinforced-polyetherimide,carbon reinforced-polyimide, carbon reinforced-polyester, carbonreinforced-acrylonitrile butadiene styrene, glass reinforced-polyimide,glass reinforced-polyethylene terephthalate, glassreinforced-polyamide-imide, glass reinforced-nylon, glassreinforced-polyvinyl chloride, glass reinforced-polycarbonate, glassreinforced-polyphthalamide, glass reinforced-polysulfone, glassreinforced-maleimide/bismaleimide, glassreinforced-polyetheretherketone, glass reinforced-polyetherimide, glassreinforced-polyimide, glass reinforced-polyester, or glassreinforced-acrylonitrile butadiene styrene. In specific variations ofthe packing column, the column has an internal diameter of about 9 mmand a length of about 65 mm. However, it is apparent that the dimensionsof the packing column can vary without deviating from the scope of theinvention. For example, the column optionally has a diameter of 8 mm,8.1 mm, 8.2 mm, 8.3 mm, 8.4 mm, 8.5 mm, 8.6 mm, 8.7 mm, 8.8 mm, 8.9 mm,9.0 mm, 9.1 mm, 9.2 mm, 9.3 mm, 9.4 mm, 9.5 mm, 9.6 mm, 9.7 mm, 9.8 mm,9.9 mm, 10.0 mm, 10.1 mm, 10.2 mm, 10.3 mm, 10.4 mm, 10.5 mm, 10.6 mm,10.7 mm, 10.8 mm, 10.9 mm, 11.0 mm, 11.1 mm, 11.2 mm, 11.3 mm, 11.4 mm,11.5 mm, 11.6 mm, 11.7 mm, 11.8 mm, 11.9 mm, and 12.0 mm. Otherdiameters are also envisioned in the invention. Similarly, the packingcolumn optionally has a length of 60 mm, 60.5 mm, 61 mm, 61.5 mm, 62 mm,62.5 mm, 63 mm, 63.5 mm, 64 mm, 64.5 mm, 65 mm, 65.5 mm, 66 mm, 66.5 mm,67 mm, 67.5 mm, 68 mm, 68.5 mm, 69 mm, 69.5 mm, and 70 mm. Other columnlengths are also envisioned in the invention.

The alkali metal alkoxy compound or alkali metal alkyl thiolate isflowed through the packing column at a rate of about 0.5 mL min⁻¹.

Where the solubilized phosphorus is desired, after reacting the redphosphorus with the alkali metal alkoxy compound or alkali metal alkylthiolate compound, the organic solvent is evaporated off the compoundcontaining the alkali metal and polyphosphide anions. The materialcontaining the polyphosphide anions is optionally redissolved inethanol. In some variations, the material dissolved in ethanol is thenexposed to acation exchange with (Bu₄N)Cl in ethanol to produce acation-polyphosphide material, followed by redissolving thecation-polyphosphide material in acetonitrile to form free polyphosphideanions. Nonlimiting examples of free polyphosphide anions are P₅ ⁻, P₁₆²⁻, P₂₁ ³⁻, or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is a graph showing the high-resolution ESI mass spectra of a(Bu₄N)₂P₁₆ solution in acetonitrile.

FIG. 2 is a graph showing the low-resolution ESI mass spectra of a(Bu₄N)₂P₁₆ solution in acetonitrile.

FIG. 3 is an illustration of the solution-based activation pathways forred phosphorus.

FIG. 4 is a graph showing the ³¹P{¹H} NMR spectra of solutions obtainedfrom reactions of P_(red) with KOtBu in THF/DME (1:1 v/v).

FIG. 5 is a graph showing the ³¹P{¹H} NMR spectra of solutions obtainedfrom reactions of P_(red) with KOnHex in THF/DME (1:1 v/v).

FIG. 6 is a graph showing the ³¹P-³¹P COSY NMR spectrum of K₂P₁₆ inEtOH.

FIG. 7 is a graph showing the ³¹P{¹H} NMR spectrum of (TBA)₂P₁₆ inacetronitrile-d₃.

FIG. 8 is a graph showing the ³¹P{¹H} NMR spectra of reaction mixturesobtained from the activation of P_(red) with K. Symbols indicateresonances from P₅ ⁻(

), P₂₁ ³⁻(□), P₁₆ ²⁻(O), P(OEt)₃(Δ), and the external shift reference(*).

FIG. 9 is a graph showing the ³¹P{¹H} NMR spectra of reaction mixturesobtained from the activation of P_(red) with KOEt. Symbols indicateresonances from P₅ ⁻(

), P₂₁ ³⁻(□), P₁₆ ²⁻(O), P(OEt)₃(Δ), and the external shift reference(*).

FIG. 10 is an illustration showing the flow reactor diagram displayingthe nearly colorless solution of KOEt in THF/DME (a) flowing into theP_(red)-loaded reactor and the red solution of polyphosphides exitingthe column followed by collection into a round-bottom flask (b).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a partthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. As used in this specification and theappended claims, the term “or” is generally employed in its senseincluding “and/or” unless the context clearly dictates otherwise.

As used herein, “about” means approximately or nearly and in the contextof a numerical value or range set forth means±15% of the numerical.

As used herein, “red phosphorus” means a polymeric molecule ofphosphorus, wherein one bond in a tetrahedron structure is bound to anadjacent tetrahedron structure resulting in a chain-like molecule havingan amorphous structure. Red phosphorus may be formed through thermaldecomposition of white phosphorus at around 250° C. to 300° C. (482° F.to 572° F.) or by exposing white phosphorus to sunlight.

As used herein, “black phosphorus” or P_(black) refers to an allotropeof phosphorus that appears black and flakey. The allotrope comprises anorthorhombic or cubic crystalline structure having puckered sheets andwhich possess conductivity, a large direct band gap and directionalanisotropy. P_(black) is generally formed from thermal decomposition ofwhite phosphorus at elevated pressures (typically around 12,000 ATM),though other methods of synthesis, such as catalysis, are envisioned.

As used herein, “white phosphorus” or P_(white) refers to an allotropeof phosphorus having a white or light yellow color. The allotrope isvery reactive. Pwhite possesses a tetrahedral or cubic crystallinestructure.

As used herein, “alkali metal” refers to the elements lithium, sodium,and potassium, including mixtures thereof.

As used herein, “alkali metal compound” refers to a compoundincorporating an alkali metal into the structure and having the alkalimetal bound via an electrostatic bond.

As used herein, “organic solvent” to a hydrocarbon liquid that dissolvesa solid, liquid, or gaseous solute, resulting in a solution. The termorganic solvent refers to compounds that contain carbon atoms, i.e. areorganic compounds.

As used herein, “inert environment” is an environment or atmosphere thatis substantially free of oxygen. In some embodiments, the inertenvironment consists of inert gases, a mixture of inert gases, or avacuum.

As used herein, “inert gas” or “inert gases” refer to noble gases (group18).

As used herein, “polyphosphide anions” are negatively charged moleculesthat contain P—P bonds. The polyphosphide anions possess increasedsolubility compared to phosphorus allotropes or variants, such as whitephosphorus, black phosphorus, and red phosphorus.

As used herein, “grain size” refers to the average diameter of a solidmaterial when in a liquid medium.

A facile solution-based method for activating red phosphorus withnucleophilic reagents, yielding soluble polyphosphides by heating atreflux under mild conditions is provided. Moreover, by employing asolution-phase activator, this reaction was adapted to a flow-chemistryprocess to afford continuous production of soluble polyphosphides.

Activation of P_(red) was initiated by surveying approaches that werepreviously used for the activation of P_(white) to form polyphosphidesby using alkali metals (Guérin & Richeson, Facile Interconversion ofPolyphosphides and Formation of a New Polyphosphide Anion. Inorg. Chem.1995 May; 34(11): 2793-2794), LiPH₂, (Baudler & Exner, Chemistry ofphosphorus. 121. Dilithium hexadecaphosphide (Li₂P₁₆) via nucleophiliccleavage of white phosphorus with lithium dihydrogenphosphide. Chem.Ber. 1983, 116(3), 1268-1270; Baudler, et al., Chemistry of phosphorus.157. Dilithium hexadecaphosphide, Li₂P₁₆: preparation from Li₂HP₇ andstructure determination by ³¹P NMR spectroscopy. Z. Anorg. Allg. Chem.1985, 529, 7-14; Baudler & Düster, Contributions to the chemistry ofphosphorus: disodium hexadecaphosphide-preparation via Cleavage of Whitephosphorus with sodium. Z. Naturforsch. B. 1987 March, 42(3), 335-336;Baudler, et al., Chemistry of phosphorus. 162. MI₃P₁₉ (MI=lithium,sodium, potassium), the first salts with nonadecaphosphide(3-) ions. Z.Anorg. Allg. Chem. 1986, 534, 19-26), or sodium naphthalenide (Cummins,et al., The stannylphosphide anion reagent sodium bis(triphenylstannyl)phosphide: synthesis, structural characterization, and reactions withindium, tin, and gold electrophiles. Inorg. Chem. 2014 Apr. 7;53(7):3678-3687).

Earlier reports described the activation of P_(white) by variousnucleophilic reagents (Brown, et al., The reaction of thiolates withelemental phosphorus. Phosphorus Sulfur. 1978, 5(1), 121-122; Brown, etal., Reaction of Elemental Phosphorus with Alkanethiolates in thePresence of Tetrachloromethane. J. Chem. Soc. Perkin Trans. 1. 1979, 7,1799-1805; Giffin & Masuda, Reactivity of white phosphorus withcompounds of the p-block. Coord. Chem. Rev. 2011, 255(11-12), 1342-1359;Scheer, et al., P₄ activation by main group elements and compounds.Chem. Rev. 2010 Jul. 14; 110(7):4236-4256). A similar approach wastested to activate P_(red), which is commonly thought of as a more inertphosphorus source.

Example 1

All manipulations with air- and moisture-sensitive compounds wereperformed under an inert-gas atmosphere by using standard Schlenktechniques or an Ar-filled glovebox. Red phosphorus (99.999%, AlfaAesar) and all anhydrous solvents (Sigma-Aldrich) were used as received.KOEt was freshly prepared from EtOH and K metal (99.5%, Sigma-Aldrich)and dried under vacuum overnight. Reaction mixtures were sampled foranalysis by ³¹P NMR spectroscopy in air-free NMR tubes. Conversions weredetermined indirectly by isolation and weighing of unreacted P_(red)starting material. Elemental analyses (C, H, N, P) were performed byMidwest MicroLab, LLC.

Activation of P_(red) was undertaken using small pieces of K (30 mg, 0.8mmol) and P_(red) (50 mg, 1.6 mmol), suspended in DME/THF (3 mL; 1:1v/v), as seen in FIG. 3. The mixture was heated under reflux for 3 h. Byreacting P_(red) with potassium metal in refluxing THF/DME (1:1 v/v;DME=1,2-dimethoxyethane), orange/red solutions were obtained thatcontained a mixture of soluble polyphosphide anions, specifically P₅ ⁻,P₁₆ ²⁻, and P₂₁ ³⁻, according to the ³¹P NMR spectra, as seen in FIG. 8.The chemical shift values of the corresponding resonance signalscoincide well with previously reported values.

The resulting bright-orange solution was allowed to cool to RT and wasthen evaporated to dryness under reduced pressure. The dark-red residuewas dissolved in anhydrous EtOH to yield a dark-red solution, whichcontained exclusively K₂P₁₆ (as determined by ³¹P NMR spectroscopy).

Next, a solution of potassium ethoxide (KOEt) was used, which led to aremarkable activation of P_(red) in less than 2 h, as seen in FIG. 9.The activation by KOEt proceeded successfully in a variety of organicsolvents, with the exception of low-boiling hydrocarbons, as seen inTable 1. The resonance signals detected in the ³¹P NMR spectrum wereassigned to the P₅ ⁻, P₁₆ ²⁻, and P₂₁ ³⁻ ions,

TABLE 1 Reaction of P_(red) with KOEt in various solvents. Solvent T [°C.] Reaction time [h] Polyphosphides produced Pentane 36.1 24 Noreaction Hexane 69 24 No reaction MeCN 81.6 2 P₁₆ ²⁻, P₂₁ ³⁻, P₅ ⁻THF/DME (1:1) 85 2 P₁₆ ²⁻, P₂₁ ³⁻, P₅ ⁻ DMF 153 24 P₅ ⁻ (major), P₁₆ ²⁻,P₂₁ ³⁻ DMSO 189 0.25 P₅ ⁻ (major), P₁₆ ²⁻, P₂₁ ³⁻

The reaction conditions were tested using different nucleophiles andreaction conditions in order to achieve the conversion of the P_(red)allotrope into various polyphosphide anions. Alkali metal-based oxyalkyland thiolate compounds were found to effectively dissolve P_(red), asseen in Table 2.

TABLE 2 Effect of various activators on the dissolution of P_(red).Activator Solvent Temperature, ° C. Reaction time, h Na- and K- MeCN81.6 2 methoxides and THF/DME 85 2 ethoxides (1:1v/v) DMF 153 0.25 DMSO189 0.25 K-t-butoxide THF/DME 85 24-48 and n- (1:1 v/v) hexoxide;Na-thiolates

The activation of P_(red) with redox-inactive KOEt in THF/DME isdrastically different from common approaches used for the synthesis ofpolyphosphides. However, the nucleophilic attack that initiates acascade of disproportionation reactions and rearrangements was proposedas the mechanism for the conversion of P_(white) to organophosphoruscompounds by p-block nucleophiles (Scheer, et al., P₄ activation by maingroup elements and compounds. Chem. Rev. 2010 Jul. 14;110(7):4236-4256).

Solution ³¹P NMR spectra were obtained on a Bruker AVANCE III 600spectrometer operating at frequencies of 600.13 MHz for ¹H and 242.96MHz for ³¹P with a 5-mm broadband probe. The chemical shifts werereferenced to 85% phosphoric acid (H₃PO₄) at 0 ppm. Each sample wasprepared under inert atmosphere by sampling 0.45 mL of reaction mixture(or redissolving solid samples in appropriate solvents), filtering, anddispensing into an air-free NMR tube followed by insertion of a sealedcoaxial insert (85% H₃PO₄ in D₂O) for locking and referencing purposes.

Indeed, the ³¹P NMR spectrum of the crude mixture obtained from theactivation of P_(red) with KOEt contains an intense signal at δ=138 ppm,as seen in FIG. 9, which corresponds to P(OEt)₃, the main byproduct ofthe disproportionation reaction:

The observation of the formation of the P(OEt)₃ byproduct, which was notdetected in the activation of P_(red) with K metal, as seen in FIG. 8,corroborates the proposed nucleophilic initiation of the transformationas described by the equation above.

To further probe the activation mechanism, the effects of thenucleophile strength, KOR, was investigated by varying the length andbulk of the alkyl substituent. As the R group was changed from ethyl ton-hexyl, the reaction rate decreased substantially; full conversion ofP_(red) was achieved in 12-24 h for R=n-hexyl, whereas less than 2 hwere required with R=ethyl. Full conversion was never reached forR=t-butyl, as judged by the observation of unreacted P_(red) in thereaction vessel even after 96 h. These results also provide strongsupport for the nucleophilic activation of P_(red), as seen in FIGS. 4and 5.

The crude mixture of polyphosphides obtained by the reaction of P_(red)with KOEt in THF/DME was evaporated to dryness. The residue wasredissolved in EtOH, and the mixture was filtered to afford a dark-redsolution. This solution was found to contain exclusively K₂P₁₆, asconfirmed by the 2D ³¹P-³¹P COSY NMR spectrum, as seen in FIGS. 4 and 5.The P₁₆ ²⁻ polyphosphide ion was successfully crystallized by cationexchange with (Bu₄N)Cl in EtOH. The X-ray crystal structuredetermination confirmed the formation of (Bu₄N)₂P₁₆. The compound can beredissolved in acetonitrile, with the P₁₆ ²⁻ polyanion remaining intact,as judged by the ³¹P NMR spectrum, as seen in FIG. 7. In this solution,a protonated [HP₁₆]⁻ ion (m/z 496.6) was also detected usingnegative-mode electrospray ionization mass spectrometry (ESI-MS).

The ESI-MS data were collected with a JEOL AccuTOF Time-of-Flight massspectrometer. Solutions of (Bu₄N)₂P₁₆ in acetonitrile were analyzed innegative ionization mode. The instrument was calibrated with a mixtureof SDS and sodium taurocholate for high-resolution mass determination.The dominant peak at mass-to-charge ratio (m/z) 496.5865 was assigned to[HP₁₆]⁻ molecular ion (predicted m/z 496.5880). Along with the [HP₁₆]⁻molecular ion, peaks corresponding to fragmentation ions were alsoobserved, as seen in FIGS. 1 and 2.

Example 2

All manipulations with air- and moisture-sensitive compounds wereperformed under an inert-gas atmosphere by using standard Schlenktechniques or an Ar-filled glovebox. Red phosphorus (99.999%, AlfaAesar) and all anhydrous solvents (Sigma-Aldrich) were used as received.KOEt was freshly prepared from EtOH and K metal (99.5%, Sigma-Aldrich)and dried under vacuum overnight. Reaction mixtures were sampled foranalysis by ³¹P NMR spectroscopy in air-free NMR tubes. Conversions weredetermined indirectly by isolation and weighing of unreacted P_(red)starting material. Elemental analyses (C, H, N, P) were performed byMidwest MicroLab, LLC.

Synthesis of (Bu₄N)₂P₁₆: Solid KOEt (136 mg, 1.6 mmol) was dissolved inDME/THF (3 mL; 1:1 v/v) and P_(red) (50 mg, 1.6 mmol) was added to thesolution. The suspension was heated to reflux for 2 h. The resultingbright-orange suspension was allowed to cool to RT and was thenevaporated to dryness under vacuum. The dark-red residue was dissolvedin anhydrous EtOH to form a dark-red solution, which containedexclusively K₂P₁₆ (as determined by ³¹P NMR). Following cation exchangewith (Bu₄N)Cl in EtOH, the P₁₆ ²⁻ polyanion was isolated as thebrown-red salt (Bu₄N)₂P₁₆. Yield=62 mg (63%). HRMS (ESI⁻): m/z calcd forthe [HP₁₆]⁻ ion 496.5880; found 496.5865. ³¹P{¹H} NMR ([D₃]MeCN, 20° C.,242.96 MHz): δ=60, 39, 3.23, −37, −134, −175 ppm. A satisfactoryelemental analysis could not be obtained for this material, presumablybecause of the extreme sensitivity of the product.

Example 3

The use of shelf-stable P_(red) instead of highly reactive P_(white)allows was analyzed as a scalable, new reaction through flow-chemistryapproaches. Based on work with solid reagents packed in flow reactors(Opalka, et al., Continuous proline catalysis via leaching of solidproline. Beilstein J. Org. Chem. 2011, 7: 1671-1679; Opalka, et al.,Continuous synthesis and use of N-heterocyclic carbene copper(I)complexes from insoluble Cu₂O. Org. Lett. 2013 Mar. 1, 15(5), 996-999;Longstreet, et al., Investigating the continuous synthesis of anicotinonitrile precursor to nevirapine. Beilstein J Org Chem. 2013 Nov.20; 9: 2570-2578; Alonso, et al., Continuous synthesis of organozinchalides coupled to Negishi reactions. Adv. Synth. Catal. 2014, 356(18),3737-3741), rapid conversion of P_(red) to form soluble polyphosphideswas tested achieved using a packed-bed method. To this end, P_(red) wasloaded as a packed bed in a stainless steel column.

All manipulations with air- and moisture-sensitive compounds wereperformed under an inert-gas atmosphere by using standard Schlenktechniques or an Ar-filled glovebox. Red phosphorus (99.999%, AlfaAesar) and all anhydrous solvents (Sigma-Aldrich) were used as received.KOEt was freshly prepared from EtOH and K metal (99.5%, Sigma-Aldrich)and dried under vacuum overnight. Reaction mixtures were sampled foranalysis by ³¹P NMR spectroscopy in air-free NMR tubes. Conversions weredetermined indirectly by isolation and weighing of unreacted P_(red)starting material. Elemental analyses (C, H, N, P) were performed byMidwest MicroLab, LLC.

A coarse 14-25 mesh (1.4-0.7 mm) powder of P_(red) (5.7 g, 2.25 mol) wasloaded as a packed bed in a stainless steel column (internal diameter=9mm, length=65 mm) in ambient atmosphere. While other sized particlesworked, smaller particles could lead to inconsistent flow profiles orchanneling of the reagent solution around the packed bed and occasionalclogging. A Phoenix flow reactor (ThalesNano) was used to apply heat(80° C.) and pressure (8 bar). After packing the column, the column wasflushed with argon. A solution of KOEt (0.38 M) in THF/DME (1:1 v/v) wasdriven through the packed bed with micro HPLC pumps (ThalesNano) at arate of 0.5 mL min⁻¹.

As seen in FIG. 10, the flow-reactor diagram began with a nearlycolorless solution of KOEt in THF/DME flowing into the P_(red)-loadedreactor, as seen in the inset (a), and upon reaction with P_(red)resulted in the dark-red solution of polyphosphides exiting the column,followed by collection into a round-bottom flask, as seen in the inset(b). After 5 h of continuous operation, 150 mL of 0.03 M polyphosphidesolution (the concentration calculated per P atom), identified as adark-red solid, was isolated by removing the volatile compounds underreduced pressure.

The solution was collected under an inert atmosphere and sampleddirectly for characterization by ³¹P NMR spectroscopy, which confirmedthe presence of the same mixture of polyphosphides as detected in, asseen in FIGS. 8 and 9. The solvent was removed under reduced pressure toyield solid products, which could be redissolved in EtOH to yield asolution of pure K₂P₁₆.

The solid was readily soluble in EtOH and yielded a spectroscopicallypure solution of K₂P₁₆ in nearly quantitative yield. ³¹P{¹H} NMR (EtOH,85% H₃PO₄ external reference, 20° C., 242.96 MHz): δ=60, 40, 4.69, −37,−134, −174 ppm.

In conclusion, a new top-down approach for the activation of redphosphorus was demonstrated to afford a fast and convenient synthesis ofsoluble polyphosphides using shelf-stable reagents and common organicsolvents. The mild nucleophilic activation of P_(red) with KOEt is instriking contrast to the previously reported activation of P_(red) whichrequired the use of strong reductants, specifically alkali metals. Thisnucleophilic activation was also adapted to a flow-chemistry reactor,providing a method for the continuous multigram synthesis ofpolyphosphides. The methodology herein can enable widespread access tothese structurally diverse species, which could lead to rapidelucidation of their reactivity pathways.

Example 4

Black phosphorus (P_(black)) is an allotrope of phosphorus that isstable at ambient temperatures and pressures. It possesses anorthorhombic structure and exhibits a black flakey structure from thepuckered structure of the phosphorus atoms. P_(black) has garneredinterest due to its ability to form thin films, along with its largeband gap (from about 0.3 to about 2.0 electron volts) and directionalanisotropy, allowing its use as field effect transistors,semiconductors, and light detectors. However, P_(black) is currentlyformed from by transformation of P_(red) under high temperature and/orpressure, and consequently contains impurities of P_(red) in thematerial. The method was used to separate P_(red) from P_(black).

KOEt was freshly prepared from EtOH and K metal (99.5%, Sigma-Aldrich)under an inert-gas atmosphere by using standard Schlenk techniques or anAr-filled glovebox and dried under vacuum overnight.

Black phosphorus (P_(black)) was synthesized from amorphous redphosphorus (P_(red)) according to a published procedure (Lange et al.,Inorg. Chem. 2007, 46, 4028-4035). After synthesis, a batch of P_(black)crystals was separated mechanically from byproducts; upon observationunder a microscope, however, unreacted deposits of P_(red) were stillvisible on the surface of BP crystals.

To remove this P_(red) contaminant, solid KOEt (136 mg, 1.6 mmol) wasdissolved in anhydrous DME/THF (3 mL; 1:1 v/v) and added to a vial (Ø 14mm) containing BP crystals (˜150 mg) under inert gas. Standard Schlenktechniques were employed for this experiment. The vial was heated toreflux for 24 h.

During reflux, the P_(red) deposits that covered the surface ofP_(black) crystals were converted to soluble polyphosphides as indicatedby the orange-red color of the resulting solution. At the same time, theP_(black) crystals remained intact. The vial was allowed to cool to roomtemperature, and the solution was evaporated to dryness under vacuum.The P_(black) crystals were washed with anhydrous ethanol (5×5 mL) underinert gas and dried under vacuum. After drying, the crystals weretransferred to a plastic container for microscopic examination.Microscopic analysis showed red phosphorus deposits, which were visibleprior to P_(red) extraction, were not visible under microscopicobservation after extraction.

Example 5

The method was used to separate P_(red) from P_(black) usingflow-chemistry approaches.

A coarse 14-25 mesh (1.4-0.7 mm) powder of P_(black) (5.7 g, 2.25 mol),containing of P_(red) was loaded as a packed bed in a stainless steelcolumn (internal diameter=9 mm, length=65 mm) in ambient atmosphere.While other sized particles worked, smaller particles could lead toinconsistent flow profiles or channeling of the reagent solution aroundthe packed bed and occasional clogging. A Phoenix flow reactor(ThalesNano) was used to apply heat (80° C.) and pressure (8 bar). Afterpacking the column, the column was flushed with argon. A solution ofKOEt (0.38 M) in THF/DME (1:1 v/v) was driven through the packed bedwith micro HPLC pumps (ThalesNano) at a rate of 0.5 mL min⁻¹.

The flow-reactor diagram began with a nearly colorless solution of KOEtin THF/DME flowing into the P-loaded reactor, and upon reaction withP_(red) resulted in the dark-red solution of polyphosphides exiting thecolumn, followed by collection into a round-bottom flask. After 5 h ofcontinuous operation, 150 mL of 0.03 M polyphosphide solution (theconcentration calculated per P atom), identified as a dark-red solid,was isolated by removing the volatile compounds under reduced pressure.

The solution was collected under an inert atmosphere and sampleddirectly for characterization by ³¹P NMR spectroscopy, which confirmedthe presence of the same mixture of polyphosphides as detected in, asseen in FIGS. 8 and 9.

The stainless steel column was then opened and the P_(black) removed.Analysis of the P_(black) indicated that the in-line column extractedthe P_(red) from the P_(black), resulting in higher purity P_(black)material. The activation of red phosphorus affords a fast and convenientmethod for the synthesis of soluble polyphosphides using shelf-stablereagents and common organic solvents, followed by removal of the solublepolyphosphides from black phosphorus. Nucleophilic activation of P_(red)with KOE was adapted to a flow chemistry reactor, providing a method forthe continuous synthesis of polyphosphides and removal of the P_(red)impurities from P_(black).

All referenced publications are incorporated herein by reference intheir entirety. Furthermore, where a definition or use of a term in areference, which is incorporated by reference herein, is inconsistent orcontrary to the definition of that term provided herein, the definitionof that term provided herein applies and the definition of that term inthe reference does not apply.

The advantages set forth above, and those made apparent from theforegoing description, are efficiently attained. Since certain changesmay be made in the above construction without departing from the scopeof the invention, it is intended that all matters contained in theforegoing description or shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention that, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A method for producing polyphosphide anionssoluble in organic solvents, comprising: providing red phosphorus or acomposition of black phosphorus and red phosphorus; providing an alkalimetal alkoxy compound or alkali metal alkyl thiolate compound suspendedin an organic solvent; wherein the alkali metal is sodium or potassium;allowing the red phosphorus and alkali metal or alkali metal compound toreact, wherein the reaction uses reflux or an in-line packed columnmethod; where the in-line packed column method comprises: providing apacking column wherein the packing column has an internal diameter ofabout 9 mm and a length of about 65 mm; loading the packing column withthe red phosphorus or composition of black phosphorus and red phosphorusin an inert environment; purging the packing column with an inert gas;setting a pressure regulator on the packing column to at least 1;heating the packing column to a preselected temperature, where thepreselected temperature is at least the boiling point of the alkalimetal alkoxy compound or alkali metal alkyl thiolate; suspending thealkali metal alkoxy compound or alkali metal alkyl thiolate compound inthe organic solvent; flowing the organic solvent through the packingcolumn; flowing the alkali metal alkoxy compound or alkali metal alkylthiolate compound in the organic solvent through the packing column at arate of about 0.5 mL min⁻¹; forming a compound containing the alkalimetal and polyphosphide anions; evaporating the organic solvent off thecompound containing the alkali metal and polyphosphide anions;redissolving the compound containing the alkali metal and polyphosphideanions in ethanol; subjecting the compounds containing alkali metal andpolyphosphide anions to a cation exchange with (Bu₄N)Cl in ethanol toproduce a cation-polyphosphide material; and redissolving thecation-polyphosphide material in acetonitrile to form free polyphosphideanions; wherein the free polyphosphide anions are P₅ ⁻, P₁₆ ²⁻, P₂₁ ³⁻,or a combination thereof.
 2. The method of claim 1, wherein the alkalimetal alkoxy compound or alkali metal alkyl thiolate compound ispotassium ethoxide, potassium methoxide, sodium ethoxide, sodiummethoxide, or sodium methyl thiolate.
 3. The method of claim 1, whereinthe organic solvent is methyl cyanide, dimethyl sulfoxide,dimethylformamide, or a combination of tetrahydrofuran anddimethoxyethane.
 4. The method of claim 3, wherein the combination oftetrahydrofuran and dimethoxyethane is at a volume to volume ratio of1:1.
 5. The method of claim 1, wherein the reflux is performed for about15 minutes to about 2 hours.
 6. The method of claim 1, wherein the redphosphorus has a grain size of about 1.4 mm to about 0.7 mm.
 7. Themethod of claim 1, wherein the packing column is stainless steel, castaluminum, A10 aluminum, A12 aluminum, stainless steel, aluminum-nickelalloy (50-50), polyimide, polyethylene terephthalate, polyamide-imide,nylon, polyvinyl chloride, polycarbonate, polyphthalamide, polysulfone,maleimide/bismaleimide, polyetheretherketone, polyetherimide, polyimide,polyester, acrylonitrile butadiene styrene, carbon reinforced-polyimide,carbon reinforced-polyethylene terephthalate, carbonreinforced-polyamide-imide, carbon reinforced-nylon, carbonreinforced-polyvinyl chloride, carbon reinforced-polycarbonate, carbonreinforced-polyphthalamide, carbon reinforced-polysulfone, carbonreinforced-maleimide/bismaleimide, carbonreinforced-polyetheretherketone, carbon reinforced-polyetherimide,carbon reinforced-polyimide, carbon reinforced-polyester, carbonreinforced-acrylonitrile butadiene styrene, glass reinforced-polyimide,glass reinforced-polyethylene terephthalate, glassreinforced-polyamide-imide, glass reinforced-nylon, glassreinforced-polyvinyl chloride, glass reinforced-polycarbonate, glassreinforced-polyphthalamide, glass reinforced-polysulfone, glassreinforced-maleimide/bismaleimide, glassreinforced-polyetheretherketone, glass reinforced-polyetherimide, glassreinforced-polyimide, glass reinforced-polyester, or glassreinforced-acrylonitrile butadiene styrene.
 8. The method of claim 7,wherein the packing column is stainless steel and wherein thepreselected temperature is at least 80° C.
 9. The method of claim 7,wherein the stainless steel packing column is pressurized to about 8bar.
 10. The method of claim 1, wherein the temperature of the packingcolumn or a reaction solution does not exceed the critical temperatureof the organic solvent.
 11. The method of claim 1, further comprisingseparating the black phosphorus from the polyphosphide anions.